Topical pharmaceutical preparations of different types have been used for treatment of rheumatic and arthritic pain for decades. Semisolid compositions comprise plant derivatives, such as capsaicin (red hot pepper stinging substance) or turpentine (pine tar component) ointments, homeopathic extract and liniments (Opodeldoc Rus), mustard plasters, menthol rubs, essential oil balms and many others were used for a long time, mainly as local irritants. Such irritation improves local blood flow, accelerates injured tissue recovery, and switches attention from chronic pain from inflammation.
By including non-steroid anti-inflammatory drugs (NSAIDs) into ointment or cream application onto the desired allows fbr effective control of muscle and joint pain intensity. Moreover, when NSAIDs are applied topically, local drug concentration in muscle and joint tissues is significantly higher than in non-treated sites. Additionally, there is no intensive metabolism in liver (so called “first-pass effect”) because such drugs do not pass through the liver before action.
The required amount of NSAIDs is lower than an oral dose to achieve similar anti-inflammative and analgesic effects. The most common side effect of NSAIDs is serious irritation of stomach and gastro-intestinal mucosa. This is substantially diminished with local topical applications.
Topical NSAID formulations are very popular in Europe, Asia and Far East regions. Examples of compositions include Voltaren Emulgel® (Voltarol™ in UK), a 1.16% Diclofenac diethylammonium emulsion cream with isopropyl alcohol, Feldene® Gel (0.5% Piroxicam water-ethyl alcohol gel), Ibuprofen and Ketoprofen gels of different strengths (5–10%), and 1–10% Indomethacin in alcohol. DMSO-containing creams and many other formulations are widespread in many countries as OTC remedies for muscle pain, sport minor injuries, rheumatic and back pain treatment, etc.
Generally, topical NSAID preparations do not have attributable side-effects such as gastric irritation and internal bleeding. Advantageously, the compounds provide relatively fast action onset and moderate efficacy in treatment of local muscle and joint pain. The main problems of these products is low drug loading due to low solubility in the cream components. High loading can be reached by use of concentrated alcohols, i.e. ethyl alcohol, isopropyl alcohol with polyethylene glycol and propylene glycol suitable as solvents for NSAIDs. Drug loading is high and can easily reach 5–10% or greater, e.g., 5% Ibuprofen gel with isopropyl alcohol, 1% Indomethacin gel based on ethyl alcohol or even 10% Indomethacin ointment with dimethylsulfoxide.
These solvents are widely used for gel preparation, but widespread use is often limited due to the proclivity for skin irritation. A further limitation is realized in fast termination of action for gel preparations since the drug precipitates from solution subsequent to water absorption from the body tissue. Further, solvents in high concentration often irritate the skin due to drying and delipidisation and may initiate contact dermatitis and allergy. Drug, insoluble in water media and body fluids, precipitates in the upper skin layers and does not penetrate inside, seriously limiting anti-inflammatory action. Similar behavior was observed for polyethylene glycol (mixture of PEG-4000 and PEG-400) hydrophilic topical base.
Traditional hydrophobic vehicles such as fixed oils, mineral oil, petrolatum, lanolin and wax based ointments, along with emulsion creams (either O/W or W/O type) are less irritating to human skin, but these present another complication—solubility. Drug loading in such vehicles is limited by the solubility of the drug in the lipid phase. For example, the solubility of Indomethacin in olive or corn oil is below 0.2%, whereas Ketoprofen is about 1.5% and Piroxicam below 0.05%. According to Benita et al. “Submicron Emulsions as Colloidal Drug Carriers for Intravenous Administration: Comprehensive Physicochemical Characterization”, J. Pharm Sci., 1993, Nov. 82 (11), pp. 1069–79, even for low drug loading, stability of the dispersed system is questionable. A 0.1% Indomethacin submicron emulsion lost stability after 1 month storage.
Use of more polar hydrophobic compounds may help to improve solubility of NSAIDs. Tocopherol acetate, triethyl citrate, glycerin monolaurate, glycerin monooleate (Myverol™ 18–9) dissolve between 1.5 and 2 times more Indomethacin or Diclofenac (in acidic form). Nevertheless, this loading is insufficient to obtain an effective NSAID emulsion. Transdermal adhesive systems such as skin patches and plasters with Indomethacin or Diclofenac present low efficacy by the same reasoning.
A further method to increase drug solubility in the oil phase is to use highly polar compounds, miscible with named phase. Solvents such as Ethoxyethylene glycol (Transcutol™), dimethylisosorbide (DMIS), Isopropylideneglycerin (Solketal™), ethoxylated furanyl alcohol (Glucofurol™) visibly boost drug implementing in the separate hydrophobic phase. However, upon mixing with water, most of the solvent is extracted into the water and the dissolved drug precipitates immediately and almost entirely from the oil phase.
Recently developed submicron emulsions (SME) employed as a base for NSAIDs, provides very effective delivery and exert pronounced improvement for drug action in Friedman et al. (U.S. Pat. No. 6,113,921). However, low solubility of NSAIDs in a lipid phase of such emulsions leads to shortened periods of efficacy and drug precipitation from the oil phase during storage. High loading, desirable for optimal activity of topical NSAID preparation for SME is achievable only for highly lipophilic compounds, such as Naproxen, Ketoprofen or Ibuprofen with significantly lower anti-inflammatory activity.
Eutectic mixture use in topical applications is rather limited. An example is EMLA cream, developed by Astra-Zeneca. The liquid, formed by mixing two crystalline bases of local anesthetics, Lidocain and Prilocain due to eutectic formation serves as an oil phase in the cream for topical application. The cream, containing 5% of such oil phase, provides excellent stability and anesthetic action.
In view of the limitation in the anti-inflammatory drug art, there exists a need for an improved composition which overcomes the shortcomings presently encountered.